Experiment III - Buffers Object: To demonstrate the action of buffers 1 Material

Question

Experiment III - Buffers Object: To demonstrate the action of buffers 1 Materials: (2) 10ml. pipettes and (3) 5 ml pipetes M/15 Naz HPO 3% Gelatin solution distilled water methyl red; cresol red (indicator) 0.1 N HCI: 0.1 N NaOH large test tubes Procedure: Mix 7 ml. of M/15 Na,HPO, and 7 ml. /15 KH,PO4 Place 5 ml. of the phosphate mixture into each of two large tubes. To one tube add 5 drops of methyl red (red at pH 4.4 and below, yellow at 6.0 and above) and then add 0.1 N HCL, drop by drop, and determine the number of drops required to reach the red end point of the indicator. To the other test tube containing 5 ml. of the phosphate mixture, add 5 drops of cresol red (yellow at pH 7.2 and below, red at 8.8 and above) and then add 0.1N NaOH,- by drop. and determine the number of drops required to reach the red end point of the indicator Repeat this exercise using tubes containing, instead of the phosphate mixture, 5 ml. of distilled water, and secondly, 5 mL of 3% gelatin solution. Number of drops of reagent required OINHCL 0.1 NNaOH 2 Phosphate mixture: Distilled water 2 Gelatin solution: Questions: a. Define a buffer solution. b. Define an indicator c. Explain the action of the phosphate buffer mixture. d. How do you account for the action of the gelatin solution? Bio 205 page 4

Explanation / Answer

a.

1.Acidic buffer- CH3COOH (Weak acid) + CH3COONa (Salt)

2. Basic buffer- NH4OH (Weak base) + NH4Cl (salt)

b.

c. Phosphate buffer comprises of: HPO42- / H2PO4-

If acid is added to phosphate buffer, acid reacts with the mono-hydrogen phosphate to form di hydrogen phosphate.

HPO42- + H3O+ = H2PO4- + H2O

If base is added to phosphate buffer, base reacts with di hydrogen phosphate to form mono-hydrogen phosphate.

H2PO4- + OH- = HPO42- + H3O+

d. Gelatin helps to analyze protective colloid effect, especially stability of hydrophobic colloids.

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